Some computing devices, such as laptop computers, may be manufactured without wireless communication capability. Rather, they are provided with slots or similar coupling expedients into which wireless communication devices may be mated to provide the host computing device with wireless capability. The wireless communication device, referred to herein as a PC card, can be for example a PCMCIA (personal computer memory card international association) type card, and can include a transceiver and other circuitry coupled to an antenna and matable with the host device to provide wireless communication capability thereto.
FIG. 1 is an isometric view of a laptop computer 100 configured to receive a PC card 102 in a slot 104 provided in a side face 106 of the laptop computer. When inserted into the slot 104, PC card 102 makes electrical connections with the laptop computer to provide wireless communication capability thereto. PC card 102 can be considered as having two main modules—RF (radio frequency) system module 102a and antenna module 102b. RF system module 102a houses the active electronic components (not shown), such as amplifiers, modems, controllers, transceivers, and so forth, while antenna module 102b houses the one or more antennas and their related devices (not shown), possibly including matching and shunting components, couplings, feedlines, ground planes, and so forth. In the view of FIG. 1, side face 106 and slot 104 extend generally in the X direction. PC card 102 has a transverse axis (not shown) also extending in the X direction, and a longitudinal axis (not shown) extending in the Y direction. The PC card 102 is moved in the direction of its longitudinal axis, as depicted by double-headed arrow c, for coupling to or uncoupling from the laptop computer 100. The coupling is effected by way of a connector array 108. Modules 102a and 102b are generally disposed along the longitudinal axis relative to one another.
Many different types of antennas can be used with wireless communication devices such as PC card 102. Diversity antennas are very beneficial for improving the quality of the received signal in a wireless communications receiver. Typical diversity antenna systems consist of a main antenna and a diversity antenna, although there could be more than one diversity antenna. In the example of FIG. 1, the main and diversity antennas would generally be housed in antenna module 102b of PC card 102.
One benefit of diversity in such a system comes from the de-correlation of the fading between two separate antenna systems. The antennas can be spatially separated and/or use orthogonal or other dissimilar polarizations (i.e. vertical and horizontal polarizations, right and left circular polarization, etc.) During a fade, the signal strength is degraded to the point that long error bursts occur in the received signal, degrading the overall received radio throughput, among other degradations. Diversity helps alleviate this problem by having two antennas separated in space and/or polarization, providing two nearly independent receive signal channels or paths which do not experience fades in the same way (that is, they are de-correlated, or exhibit orthogonality). Thus while one antenna may experience a deep fade the other antenna may be within 3 dB of its nominal signal level. The result of this is that links with rapid fading that can go −15 dB or more below the average signal strength in a fade on a single channel system (non-diversity) but may be reduced to only −4 dB or −5 dB below the average signal strength with diversity on a statistical basis. In this example, diversity would provide an effective gain of 11 dB to 10 dB. Thus the reduced loss of signal prevents the channel from being dropped far less frequently than it would with a single deep fading channel. The diversity antenna may be separated by as little as one eighth of a wavelength and still experience a significant gain over a single channel non-diversity antenna.
Diversity antenna systems for use in high volume applications are demanding ever decreasing costs in integration and assembly. Reduction in the interconnect costs and simplification by integration and the elimination of discrete components is also a major cost reduction goal. Size reduction, while maintaining reasonable RF efficiency and isolation, is also a rigorous requirement.
For many wireless technologies, a diversity antenna is desired to be included in a very small volume where the main antenna resides, without excessive electromagnetic coupling to the main antenna. As explained above, one aim of the diversity antenna concept is to provide reception of the signal when the main antenna is situated in an area of signal cancellation due to “multi-path,” or “fading” of the signal, but the diversity antenna must not be electromagnetically coupled to the main antenna—that is, it must have a level of isolation, to meet requirements of the wireless network which electronically switch from the main signal path to the diversity signal path, depending on which path offers the better signal reception. Another reason for an isolation requirement may be to protect the diversity receiver front end components from excessive power transmitted from the main antenna. As such, one of the difficulties is to design a diversity antenna that receives the same frequency bands as the main antenna, but does not lose the received signal into the main antenna (which may be instantaneously turned off in favor of the diversity channel), instead directing the signal into the diversity channel of the radio, and not receiving excessive signal energy being transmitted by the same radio through the main antenna. The diversity antenna is intended to couple into a signal field polarization, or signal field location, that is not available to the main antenna. It is thus desired to have different antenna polarizations, antenna locations or antenna radiation patterns, or any combination of these, for the main and diversity antennas, while meeting the requirements of the overall antenna system such as size, cost, electrical performance, appearance, weight, or any other requirements specific to the application.